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What is Logic Gate ? Logic Gates Explained

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Hey friends, welcome to the YouTube channel ALL ABOUT ELECTRONICS.

So, in this video, we will learn about the logic gates.

The logic gates are a very basic building block of digital systems.

So, these logic gates are the electronic circuit which consists of one or more inputs and one

output.

Now, the inputs and outputs of these logic gates can have only two values.

That is logic '1' or the high value and the logic '0' or the low value.

Now, in reality, this high or low or the logic '1' or the logic '0' are the voltage levels.

That means the high could be 5 V, while the low could be zero volt.

And the relationship between this input and output of the logic gate is based on a certain

logic.

For example, for some logic gate, the output would be high or the logic '1' when both inputs

are high.

Or for some other logic gate, the output would be high if any one of the inputs is equal

to logic '0'.

So, in a way, these logic gates have the ability to make certain logical decisions.

And since these electronic gates can make logical decisions, so these gates are known

as the logic gates.

Now, when we interconnect or cascade these logic gates, then it is possible to perform

various logical operations.

And using these logic gates even it is possible to build the processor.

So, now let's understand about these logic gates.

So, this AND gate, OR gate, and the NOT gate are the very basic types of logic gates.

And using these three gates, it is possible to build any boolean function.

Apart from that, there are two universal gates.

That is NAND gate and the NOR gate.

So, these two gates are known as the universal gates because using any of the two gates alone,

it is possible to build any logic circuit or the boolean function.

So, apart from these universal and the basic gates, there are two more gates.

That is XOR gate and the XNOR gate.

So, one by one, let's learn about all these different logic gates.

And first, let's start with the AND gate.

So, this is the symbol of the 2-input AND gate.

Where this A and B are the inputs of this AND gate while this Y is the output.

So, this output of the AND gate will be high or the logic '1' when the both inputs are

high.

And if any of the two input is low, or the logic '0' in that case, the output of the

AND gate will be equal to 0.

So, let's understand that with the help of the truth table.

So, this truth table shows all the possible combinations of the input signal and the corresponding

outputs for those input combinations.

So, here is the truth table of the 2-input AND gate.

So, as I said earlier, each input to the logic gate can have only two values.

That is logic '0' or the logic '1'.

So, for the two different inputs, there are total 4 different possibilities.

That is either both inputs are logic '0' or both inputs are logic '1'.

And the other two possibilities are when both inputs are different.

So, as I said earlier, the output of the AND gate is logic '1', when both inputs are high.

And if any of the two input is low or the logic '0' in that case, the output will also

be equal to logic '0'.

So, let's say, this A and B are the input to this AND gate.

And Y is the output.

So, the boolean expression of this AND gate can be given as Y= AB.

That means if input A and B both are 1, then and then only, this output of the AND gate

will be equal to 1.

And if any of the two input is low or the logic '0', in that case, the output of the

AND gate will be equal to low.

So, that is the two-input AND gate.

Similarly, we can also have AND gate with more than 2 inputs.

So, here is the symbol of the 3-input AND gate.

Where this A, B, and C are the input to these AND gate.

And if we see the boolean expression of this 3-input AND gate, then this output Y can be

given as ABC.

That means whenever, all the three inputs are high then and then only, the output of

the AND gate will be equal to high.

Similarly, we can also have n-input AND gate.

So, now, let's move to the next gate.

And the next gate is the OR gate.

So, this is the symbol of the OR gate.

Where this A and B are input to this OR gate and Y is the output.

So, this output of the OR gate will be low, whenever both inputs are low.

And if any of the two input is high, in that case, the output of this OR gate will be equal

to high.

So, let's understand it with the help of the truth table.

So, here is the truth table of the OR gate.

So, for the two inputs A and B, there are total 4 different combinations.

So, in case of the OR gate, the output is low, whenever both inputs are low or logic

'0'.

And if any of the two input is high or both inputs are high in that case, the output of

the OR gate will be equal to high.

So, if A and B are the input to this OR gate, and Y is the output, then the boolean expression

of this OR gate is equal to A + B. That means if either A is 1, or B is 1, or

both are 1, in that case, the output of the OR gate will be equal to logic 1.

And if both inputs are 0, in that case, the output of the OR gate will be equal to 0.

So, that is the two-input OR gate.

Similarly, we can also have an OR gate with more than 2-inputs.

So, here is the symbol of the 3-input OR gate.

And if we see the boolean expression then it is equal to A + B + C.

That means if all the three inputs are low, then and then only, the output of the OR gate

will be equal to low.

And in any other case, the output of the OR gate will be equal to high.

So, that is the OR gate.

Similarly, the next gate is the NOT gate. which is also known as the Inverter gate.

Because in this gate, the output is the complement of the input signal.

So, if the input is high, then the output will be equal to low.

And likewise, if the input is low, then the output will be equal to high.

So, this is the truth table of the NOT gate.

So, basically, this gate inverts the logic '0' to the logic '1' and the logic '1' to

the logic '0'.

And it is very useful in implementing the different boolean function.

And this is the symbol of the NOT gate.

And if A is the input to this NOT gate then this is the boolean expression of the NOT

gate, which indicates that the output is the complement of the input signal.

So, this AND, OR, and the NOT gate are three basic gates using which it is possible to

design any logic circuit or it is possible to implement any Boolean function.

But apart from that, there are two universal gates.

And using any of the two gates alone, it is possible to design any logic circuit.

So, let's understand about this NAND gate and the NOR gate.

And first, let's start with the NAND gate.

So, this is the symbol of the two-input NAND gate.

And if you see, then it is very similar to the AND gate.

But here, there is a bubble on the output side.

So, this NAND gate is the combination of the AND gate followed by the NOT gate.

That means the output of the NAND gate, is equivalent to the output of the AND gate followed

by the NOT gate.

And its boolean expression is equal to AB- bar.

That is the complement of the output AB.

So, now, let's see the truth table of the 2-input NAND gate.

Now, we have already seen the truth table of the 2-input AND gate, right !!

So, let's say, the output of this AND gate is equal to Z.

So, this output Z is equal to 1 whenever both inputs are high.

And if any of the two inputs is low, or both inputs are low, in that case, this output

Z will be equal to 0.

Now, whenever, this input is passed through the NOT gate, then it will get inverted.

That means the output of this NOT gate will be equal to Z-bar.

So, in this Z-bar, all the zeros will get replaced by 1, and 1 will get replaced by

0 And this is the output of the AND gate, followed

by the NOT gate.

Which is equivalent to the output of the NAND gate.

So, as you can see from the truth table, when all the input to the NAND gate is equal to

1, then only the output of this NAND gate will be equal to 0.

But whenever, any of the two input is logic 0, in that case, the output of the NAND gate

is equal to 1.

So, similar to the 2-input NAND gate, we can also have NAND gate which has more than 2

inputs.

But in that case, also, the output will be low whenever all the inputs are high.

Apart from that, for all other combinations, the output of the NAND gate will be equal

to high.

Alright, so now, let's see the NOR gate.

So, this is the symbol of the 2-input NOR gate.

And as you can see, it is very similar to the OR gate.

But there is a bubble on the output side, which indicates that the output of the NOR

gate is similar to the output of the OR gate followed by the NOT gate.

So, now let's see the truth table of this NOR gate.

So, let's say, the output of this OR gate is equal to Z.

And whenever it is given to the NOT gate, then the output of the NOT gate will be equal

to Z-bar. which is the complement of the Z.

Now, we have already seen the truth table of the 2-input OR gate.

And we have seen that, when any of the two input is high in that case, the output of

the OR gate will be equal to high.

And whenever, both the inputs are logic '0' then only, the output will be equal to logic

'0'.

Now, when this output Z is passed through the NOT gate, then it will get inverted.

So, all the 1s will become 0s, and all the 0s will become 1s.

And that is the output of the NOR gate.

That means the output of the OR gate followed by the NOT gate is equivalent to the output

of the NOR gate.

And the boolean expression of this NOR gate is equal to (A+B) bar.

which indicates that, the output of the NOR gate is the complement of the OR gate.

So, as you can see from the truth table, the output of the NOR gate is 1, whenever both

inputs are 0.

So, that is the two-input NOR gate.

Similarly, we can also have a NOR gate with has more than 2-inputs.

Alright, so far we have seen three basic gates and the two universal gates.

Apart from that, there are two more gates.

That is the X-OR gate and the X-NOR gate.

But we will talk more about it in the next video.

But I hope in this video, you understood what is the logic gate and you also learned some

basics of the logic gates.

So, if you have any questions or suggestions, then do let me know here in the comment section

below.

If you like this video, hit the like button and subscribe to the channel for more such

videos.

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YouTube Transcript:
What is Logic Gate ? Logic Gates Explained